The density of integrated circuits (ICs) continues to dramatically increase due to the decrease in both the size of circuit features and components (e.g., transistors) and the pitch or distance between them. In order to develop reliable IC manufacturing processes, measure characteristic feature sizes, diagnose manufacturing defects, and perform quality control, integrated circuit manufacturers typically inspect integrated circuits or portions of integrated circuits via transmission or scanning transmission electron microscopes (TEM). As used herein, the term transmission electron microscope is intended to include scanning transmission electron microscope.
Most IC device components are structurally orthogonal, having characteristic lengths, widths, and depths. Typically, images, analytical maps are taken from one of these orthogonal directions and provide the necessary end data. Given the current size of such components, transmission electron microscopy is often the only effective way to visualize, inspect and measure the features and components of IC devices. Currently, such measurements are made from the inspection of thin cross-sectional slices or lamellae that are extracted from the ICs. Extracted lamellae are typically 30-100 nm thick in the viewing direction (i.e., the transmitted electron beam direction) but can be as thin as 10 nm or below for current technology nodes, and have a cross-sectional area between about 4 um2 and 16 um2. Various problems can occur when trying to extract thin lamellae from processed Si wafers including warping, bending, over-milling, amorphisation and so-called curtaining. These problems can, in turn, lead to poor IC device characterization, such as poor measurement of the characteristic sizes of IC device features or component. Moreover, since IC device features and components are three dimensional, their proper characterization requires inspection in all three dimensions, often requiring separate lamellae to be extracted in three different viewing directions. The separate lamellae in different orientations are extracted from different instances of the same feature, such as different one of identical memory cells.
TEM samples are viewed by transmitting a beam of electrons through the sample and detecting the transmitted electrons on the opposite side to form the image. The beam is typically orthogonal to the face of the lamella. In electron tomography, the sample (or beam) is tilted through a series of tilt angles, with an image formed at each of the different tilt angles, to provide data that can be used to mathematically reconstruct a three dimensional image. Because it is impossible to obtain a complete 180 degree tilt series of a lamella, a pillar-shaped sample is sometimes used for electron tomography. A pillar shaped sample has the disadvantage of having non-uniform thickness across the sample when viewed from any direction.